Metal softening process and product thereof



METAL SOFTENING PROCESS AND PRODUCT THEREOF Filed March 28, 1966 FIG. 10

Jan. 7, 1969 Ds, JR" ET AL Sheet FIG. 2

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Jan. 7, 1969 D8. FIELDS, JR., ETAL METAL SOFTENING PROCESS AND PRODUCT THEREOF Filed March 28, 1966 Sheet 2 of2 FIG. 7

$2 2 EIE H E Z 1 g FORMING TIMEMINUTE$ FORMING TIME MINUTES FIG.6

I EUTEGTOII) |00MPOSITION ALUMINUM CONTENT-N: BY WEIGHT FORMING TIME MINUTES United States Patent 3,420,717 METAL SOFTENING PROCESS AND PRODUCT THEREOF Davis S. Fields, Jr., and Daniel L. Mehl, Lexington, Ky.,

assignors to International Business Machines Corporation, Armonk, N.Y., a corporation of New York Filed Mar. 28, 1966, Ser. No. 537,939 US. Cl. 148-115 13 Claims Int. Cl. C22t 1/04 This process significantly reduces the high temperature strength level of a superplastic zinc-aluminum eutectoid alloy. Metal prepared by this process is formable with less force and in less time than the same material lacking such preparation.

In the US. patent application Ser. No. 445,188, now Patent No. 3,340,101 entitled, Thermoforming of Metal, filed Apr. 2, 1965, by Davis S. Fields, Jr., Daniel L. Mehl, and Bernard F. Addis, a tension forming process was disclosed for shaping metals exhibiting a substantial strain rate sensitivity. Strain rate sensitivity m is the exponential variable in the expression wherein 11 represents stress in pounds per unit area; Erepresents strain rate in terms of length change per unit guage length per unit time; and K represents a proportionality constant which may be termed the strain rate coefficient. The numerical value of K depends upon the specific dimensions selected for the other variables.

The process disclosed in application Ser. No. 445,188 was characterized by a distinctive relationship between stress and strain rate, witness applied load and forming time, not exhibited in conventional metal forming processes. While the existence of a significant strain rate sensitivity m is responsible for the existence of this distinctive relationship, and determines the ultimate elongation to which a part may be subjected without fracture, we have found that the strain rate coefficient K is most significant in determining the strength level of the material and hence the primary expense factors of force and time required to form a part. In addition to the basic economic considerations of tooling and other equipment size and throughput, the reduction of deformation loads permits a greater degree of forming precision by minimizing tool deflections.

Accordingly, it has been an object of our invention to devise and develop a process for conditioning the eutectoid metal alloy comprising nominally 78% zinc, 22% aluminum by Weight, to substantially reduce its strain rate coefficient K at forming temperature.

Another object of our invention has been to investigate the effect of typical expected variations in our process to permit the generalization necessary to practical utilization of this process on other alloys related functionally to the pure zinc-aluminum eutectoid, and to permit predictable variations in the process itself.

These and other objects of our invention will be more apparent to those skilled in the art from the following more particular description of our invention wherein reference is made to the accompanying drawings, of which:

FIGURES 1(a), 1(b) and 1(0) are schematic views of the principal steps of our process;

FIGURE 2 is a data plot of true stress 0' vs. true strain rate 5, for differently preconditioned materials;

FIGURE 3 is a data plot of forming depth vs. forming time for differently preconditioned materials when subjected to a standardized part-forming operation involving biaxial tension;

FIGURE 4 is a data plot of forming depth vs. forming time similar to FIGURE 3 from tests of materials pre- 3,420,717 Patented Jan. 7, 1969 ice conditioned at different temperatures t demonstrate the optimum working temperature and the effect of variations therefrom;

FIGURE 5 is a cross plot of data taken from the curve end points of FIGURE 4, more vividly illustrating the significance thereof;

FIGURE 6 is a comparative data plot of instantaneous load as produced by a standard applied strain rate to specimens of varying zinc-aluminum content.

FIGURES 7 and 8 are data plots of forming depth vs. forming time similar to FIGURE 3 from tests to other alloys differing from the substantially pure eutectoid to demonstrate the beneficial effect of our conditioning process thereon.

Our process essentially involves the following steps as schematically shown in FIGURES 1(a), 1(b) and 1(0).

First, providing a homogenous body 10 of the eutectoid alloy stock comprising essentially 78% zinc, 22% aluminum, by weight that has been held, or solution heat treated, in an oven 11 at a temperature above the eutectoid invariant temperature near 530 for an adequate period of time to assure uniform high temperature face centered cubic structure. Attention is called to the aluminum-zinc phase diagram published in Metals Handboo copyright 1948, by the American Society of Metals, page 1167, for a more general understanding of the temperature and composition factors involved. Tentative modifications to this diagram, based on recent work by Goldak and Parr, Journal of the Institute of Metals, 19631964, vol. 92, page 230, do not affect present considerations.

Second, cooling the body 10 to a temperature substantially below the eutectoid invariant at a rate adequate to cause the material to be superplastic when later brought to forming temperatures; for example, by quenching the body in Water bath 12 with agitation.

Third, working the body, as by rolls 13, at relatively low temperatures, i.e., below 400 F. and optimally at about 300 F. We are aware of the exothermal reaction that occurs after the quench; however, tests show that substantial softening at forming temperature is obtained whether the working step occurs before or after this reaction.

By this process we have successfully produced stock material 10' having the highly desirable but unexpected property of low strength at superplastic forming temperatures as compared with material prepared without the working step. The conditioned stock material is identifiable by its substantially reduced strain rate coefficient K at forming temperatures. The decreased strength level of the material permits improvement of the final forming process, either by reducing the loads required, the time required, or some combination of these two primary expense factors.

The beneficial effect of our process on the zinc-aluminu-m eutectoid is demonstrated by the data plotted in FIGURES 2 and 3. FIGURE 2 is a graphical log-log representation of data obtained from uniaxial tensile testing at 520 F. of specimens having different degrees of low temperature working, when tested over a wide range of strain rates More specifically, curve 20 represents the tensile response of a standardized specimen having no low temperature working after the quench stage 12. Curves 21, 22 and 23 represent data taken from standardized specimens having, respectively, 25, 50 and reduction by low tem perature working in accordance with our process. The

significance of the data represented in FIGURE 2 can be seen by considering a line of constant stress, such as line 24. For a given stress it can be seen that the strain rate increases and forming time decreases by nearly a factor of 5, for highly worked material (curve 23) as compared with the unworked material (curve Similarly, a line of constant strain rate, such as 25, indicates the span of stress variation required to produce a given strain rate in the differently processed materials. For unworked material (curve 20), a considerably higher stress level is required to produce the selected strain rate than is required to deform the highly worked material (curve 23).

The nearly parallel relationship between the curves 20-23 demonstrates the regularity and predictability of our discovered phenomena. This relationship also demonstrates the absence of significant effect by our process on the exponential strain rate sensitivity factor m, as represented by the slope of the curves.

FIGURE 3 is a data plot, graphically illustrating the following practical part forming demonstration which had been performed. Four sheet specimens were taken from the same section of a common melt provided in the form of reroll stock of an alloy comprising 78% zinc, 22% aluminum, by weight to an accuracy of 99.0% purity. The material for each sheet was rolled at 620 F. to a thickness that would permit various degrees of later low temperature working to an ultimate standard thickness of 0.050 inch. All sheets were solution heat-treated at about 600 F. for approximately one hour and then quenched in water with agitation to produce an essentially equal metallurgical state. One sheet was employed as a control and was not worked further after the quench. Each of the remaining sheets was rolled at about room temperature to reduce its thickness by 25, 50 and 75 percent, respectively, producing final specimens of 0.050 inch thickness in each case. Each specimen was placed in a heated die, constructed like that described in the aforementioned application Ser. No. 445,188, and brought to a uniform temperature of 520 F. in a standardized period of sixteen minutes. A pneumatic load by way of a 14.7 p.s.i. vacuum was applied to each specimen. The time and center point deflection data plotted in FIGURE 3 was recorded during each test.

The response of the control specimen is plotted as curve in FIGURE 3. The response of the specimens worked by 25, and 75% thickness reduction is plotted as respective curves 31, 32 and 33. Note that the control specimen (curve 30) required a forming time of 3.4 minutes for the center point to reach the bottom of the die. The specimen reduced by 50% (curve 32) required 1.2 minutes for total deflection. The specimen reduced by 75% (curve 33) required only 1.1 minutes for total deflection. It can be seen that good correspondence exists between FIGURES 2 and 3. It also can be seen that the effect of our working step on the strength reduction at forming temperature decreases with further working.

Working temperature The effect of working temperature variations on the softening results of our process is demonstrated by the data plotted in FIGURE 4 and cross-plotted in FIG- URE 5.

Data plotted in FIGURE 4 was obtained by preparing six test sheet specimens of the zinc-aluminum euctectoid from the same part of the same melt by 'hot rolling ingot (above 600 F.) to 0.100 inch, solution heat treating the specimens for one hour at 600 F. and quenching the specimen in water, with agitation. The specimens were individually heated a selected temperature (100, 200, 300, 400, 500, 600 F.) and rolled to 0.050 inch, a deformation of 50%. After rolling, each specimen was quenched. The rolling required several passes and the specimens were returned to the heating oven between passes to maintain as nearly a constant temperature as possible. The specimen rolled at 600 F. was considered a control since this temperature is above the eutectoid invariant. The control specimen was given the same rolling history as the test specimens, but as expected, it behaved as if all rolling had occurred prior to the first quench.

The specimens were tested as described in connection with the-data of FIGURE 3 and produced formation curves similar thereto. The curves are identified by their respective rolling temperatures.

The significance of this data is even more vividly demonstrated in FIGURE 5 in which the end point forming times from FIGURE 4 have been plotted against working temperature. By comparison with the control data, it can be noted that a beneficial effect is obtained over a wide range of working temperatures, and that for the particular composition tested, a maximum effect was obtained in the neighborhood of 300 F. Aside from the basic beneficial effect of working at above room temperature, it is significant in itself that working can be performed above room temperature without detriment. For example, extraneous conditions may make room temperature rolling diflicult, if not impossible.

Material composition The material composition can vary, as with random impurities, significant alloy additions, or off-eutectoid composition and significant softening effect is still obtained by processing in accordance with our invention.

For further demonstration, we have performed comparative tests on specimens of varying composition. Each composition was tested with and without 50% preforming deformation.

FIGURE 6 shows the effect on the benefits of our process of relative zinc-aluminum variation over a wide range around the eutectoid (78%22%) composition. Curve 60 plots the instantaneous load for a tensile specimen subjected to a standard strain rate for various compositions, but without previous low temperatures working according to our invention. Curve 61 plots the forming load for the same compositions, as curve 60, but with 50% low temperature deformation after quenching. The continued benefit of our process over this wide range of composition variation is manifest.

FIGURES 7 and 8, respectively, show the continued benefit of our process in the presence of small but significant amounts of magnesium and manganese. FIG- URE 7 shows the response of comparative 50% low temperature worked (curve 70) and unworked (curve 71) test specimens, containing 0.02% mg. by weight, tested as described in connection with FIGURE 3. FIGURE 8 shows the response of comparative 50% low temperature worked (curve 80) and unworked (curve 81) test specimens containing 0.050% Mn. These results are quite significant in demonstrating the effectivenes of our process on other compositions, especially considering that these selected test elements are known to drastically affect the kinetics of phase decomposition in zinc-aluminum alloys.

It will be appreciated by those skilled in the art that we have discovered and developed a process for substantially enhancing the economic properties of the zincaluminum eutectoid as a superplastic structural metal. The data we have presented herein demonstrates the breadth of our invention by revealing in a general sense both optimum factors such as the rolling temperature for maximum effect and the lack of sensitivity as to the operation of the process to all the basic factors. On the other hand, it can be reasonably predicted from the data herein presented that variation of any factor can produce a varied degree of result. By moving from the optimum temperature of about 300 F. it can be expected that beneficial effect will be reduced in the pure zinc-aluminum eutectoid.

It will be recognized that apparatus is not an essential consideration to our invetnion, and that the various steps can be performed by different equipment. For example,

heating, while conventionally performed in an enclosed oven can with equal etficiency, be performed in a heated press designed to provide the working desired. Quenching can be accomplished by spraying as opposed to dunking and the working can be accomplished by extrusion, forging, etc., as well as rolling.

Accordingly, it will be seen that modifications, additions, deletions, etc., can be made to our process as specifically disclosed without necessary departing from the spirit and scope of our invention which is limited only by the appended claims.

We claim:

1. The process of preparing stock of the eutectoid alloy nominally comprising by weight 78% zinc, 22% aluminum, comprising the steps of:

solution heat treating said stock at a temperature between its eutectoid temperature and its solidus temperature for a period of time adequate to obtain a uniform structure throughout said stock, quenching said stock, and

thereafter working said stock at a temperature below 2. A process as defined in claim 1 wherein said working step is performed at a temperature of about 300 F.

3. The process as defined in claim 1 wherein said working step is accomplished by rolling.

4. A method of making metal forms comprising the steps of:

providing a body of stock prepared in accordance with the process defined in claim 1,

heating said body to a temperature just below its eutectoid temperature, and

forming said body while at temperatures just below its eutectoid temperature.

5. A method of making metal forms as defined in claim 4 wherein said body is provided in sheet form and said forming step comprises at least partially the step of applying a pneumatic load thereto to cause a substantial biaxial tensile deformation thereof.

6. A method of making metal forms comprising the steps of:

providing a body of stock prepared in accordance with the process defined in claim 2,

heating said body to a temperature just below its eutectoid temperature, and

forming said body while at temperatures just below the eutectoid temperature.

7. A method of making metal forms comprising the steps of:

providing a body of stock prepared in accordance with the process defined in claim 3,

heating said body to a temperature just below its eutectoid temperature, and

forming said body while at temperatures just below the eutectoid temperature.

8. A method of making metal forms as defined in claim 7 wherein said body is provided in sheet form and said forming step comprises at least partially the step of applying a pneumatic load thereto to cause a substantial biaxial tensile deformation thereof.

9. A body of metal alloy stock of the eutectoid comprising nominally 78% zinc, 22% aluminum by weight, said body being in a state resulting from processing in accordance With the process defined in claim 1, said body being characterized by exhibiting a substantially reduced strain rate coeflicient at temperatures just below its eutectoid temperature, as compared to a body of the same alloy similarly processed but without said working step.

10. A body of metal alloy stock of the eutectoid comprising nominally 78% zinc, 22% aluminum by weight, said body being in a state resulting from processing in accordance with the process defined in claim 2, said body being characterized by exhibiting a substantially reduced strain rate coefiicient at temperatures just below its eutectoid temperature, as compared to a body of the same alloy similarly processed but Without said working step.

11. A body of metal alloy stock of the eutectoid comprising nominally 78% zinc, 22% aluminum by weight, said body being in a state resulting from processing in accordance with the process defined in claim 3, said body being characterized by exhibiting a substantially reduced strain rate coeflicient at temperatures just below its eutectoid temperature, as compared to a body of the same alloy similarly processed but without said rolling step,

12. The process of conditioning stock of the eutectoid alloy nominally comprising by weight 78% zinc, 22% aluminum in a state produced by a solution heat treatment at a temperature between its eutectoid temperature and its solidus temperature for a period of time adequate to obtain a uniform structure throughout and followed by quenching, the improved treatment step comprising working the stock at a temperature below 400 F.

13. A process as defined in claim 12 wherein said working step is performed at a temperature of about 300 F.

References Cited UNITED STATES PATENTS 3,340,101 9/1967 Fields et a1 14811.5

L. DEWAYNE RUTLEDGE, Primary Examiner.

W. W. STALLARD, Assistant Examiner.

US. Cl. X.R. 

1. THE PROCESS OF PREPARING STOCK OF THE EUTECTOID ALLOY NOMINALLY COMPRISING BY WEIGHT 78% ZINC, 22% ALUMINUM, COMPRISING THE STEPS OF : SOLUTION HEAT TREATING SAID STOCK AT A TEMPERATURE BETWEEN ITS EUTECTOID TEMPERATURE AND ITS SOLIDUS TEMPERATURE FOR A PERIOD OF TIME ADEQUATE TO OBTAIN A UNIFORM STRUCTURE THOUGHOUT SAID STOCK, QUENCHING SAID STOCK, THEREAFTER WORKING SAID STOCK AT A TEMPERATURE BELOW 400*F. 